1. Field of the Invention
The present invention relates to a differential gear having a differential limiting function.
2. Description of the Related Art
Differential gears with a differential limiting function (limited slip differential) are used as differential gears in automobiles, which, when the wheels on only one side of the automobile are slipping, transmit sufficient torque to the wheels on the other side via clutch means. This type of differential gear is used widely for the purpose of preventing, when wheels on one side from slipping on ice, snow, or mud, the hindered transmission of torque to the wheels on the other side, and preventing the swerve phenomenon from occurring when the automobile is turning or starting abruptly.
For example, the differential gear 100 as shown in FIG. 11 is described in Japanese Patent Application Laid-Open No. S55-27980 and Japanese Patent Application Laid-Open No. 58-221046. This differential gear 100 comprises: a pair of pressure rings 103 accommodated in the differential case 101 such that they can move freely in the directions of axles 102 but cannot rotate relatively; a pinion shaft 104 that is placed at right angles to the axles 102 with the end thereof being held between the pressure rings 103; a pinion gear 105 provided rotatably on the pinion shaft 104; a pair of side gears 106, provided at the ends of the left and right axles 102 such that they cannot rotate relatively, and that sandwich the pinion gear 105 between them and engage with the pinion gear on the opposite sides thereof; operation means 108 for operating the pressure rings 103 in such a direction as to increase the clearance 107 between the pressure rings 103 in accordance with an increase in relative torque between the pinion shaft 104 and differential case 101 in a differential state; and clutch means 109 that are provided on the opposite sides, in the directions of the axles, of the pressure rings 103 and that are operated by the pressure rings 103 to limit the differential between the left and right axles 102 as the clearance 107 becomes greater.
Various forms of the operating means 108 have been proposed. One of the means adopted is a means, for example, in which a square cam part 110, one diagonal line of which is set in the directions of axles, is formed at the end of the pinion shaft, and in which a substantially V-shaped cam groove 111 is formed in the pressure rings 103 to engage with the cam part 110.
Also, in this differential gear 100, a relative torque arises between the pinion shaft 104 and differential case 101 and widening of the clearance 107 between the pressure rings 103 using the operating means 108 causes the clutch means 109 to operate and the differential between the left and right wheels to be limited. However, normally an elastic member 112, such as a disc spring, is provided outside the clutch means 109 along the axis so that the clearance 107 between the pressure rings 103 returns to its set gap. The differential gear is also configured so that it exerts pressure that draws the pressure rings 103 close to each other via the multiplicity of clutch plates 113 in the clutch means 109.
Basically, it is preferable that this type of differential gear 100 enters the differential state to enable a small turning circle when low speed circling is required such as when parking a car in a garage or at the side of the street, and enters the limited differential state to enable the desired functions to work with stability when the wheels on one side of the car are slipping, when the car is moving forward quickly, or when it is turning at high speed or under heavy loads.
However, in the elastic member 112 in the above differential gear 100, because the pressure rings 103 are urged to come closer to each other through clutch plates 113, when the urging force is weak, the clutch means 109 becomes easy to operate using the operation means 108 and the limited differential state occurs even when the automobile is turned at low speeds. This means that the differential state is not always stable. When the urging force is made stronger, the operation means 108 enables the differential state. However, because the clutch plates 113 are always frictionally abutted by the urging force exerted by the elastic member 112 and substantially is in a limited differential state, a stable differential state cannot be guaranteed when a automobile is made to turn a circle at low speeds. That is, there will be various problems such as that the automobile cannot make a sharp small turn when rotating at low speeds, or the so-called xe2x80x9cchatteringxe2x80x9d phenomenon arises, in which there is alternate switching between the limited slip differential and differential state and, as a result, maneuverability deteriorates and the shocks that occur with chattering adversely affect the differential gear 100 and peripheral equipment.
Therefore, it is extremely difficult to specify the urging power that should be applied. For example, in racing automobiles used for sport, maneuverability during low speed rotation is sacrificed to some degree and the urging power exerted by the elastic member 112 is set higher so that a sufficient limited differential can be obtained at high speeds and under high loads.
Also, in automobiles with transverse engines, the layout demands of the engine and peripheral components mean that the shape and capacity of the differential case are greatly restricted compared to automobiles with vertical engines. For example, as in the differential gear 120 shown in FIG. 12, the right part of the differential case 121 has had to be narrowed. It is difficult to adequately increase the area of the frictionally abutting section of the clutch means 122 and if the pressure exerted on the elastic member 123 is increased to increase the friction abutting force, a differential state is not obtained when low speed rotation is required. This means that an adequate limited differential action cannot be obtained.
The present invention provides a differential gear that can switch, as appropriate, between differential and limited differential states.
The differential gear according to one embodiment of the invention comprises: a differential case that is rotated by a driving force from the engine, around the left and right axles; a pair of pressure rings accommodated in said differential case so as to be able to move freely in the direction of the axles but unable to rotate relatively; a pinion shaft provided at right angles to the axles with an end thereof held between said pressure rings; at least one pair of pinion gears provided rotatably on said pinion shaft; a pair of side gears, each being placed at the end of said left and right axles so as to be unable to rotate relatively, that sandwich the pinion gears and engage with the pinion gears on the opposite sides thereof; a set of clutch means arranged on the opposite sides, in the direction of the axles, of said pressure rings and that can limit the differential movement between the left and right axles by limiting the relative rotation of the differential case and side gears; urging means that urges the pressure rings in such directions as to decrease the clearance between the pressure rings without use of said clutch means; and operation means that operates the pressure rings against the urging force exerted by the urging means in such directions as to increase the clearance between the pressure rings in accordance with an increase in the relative rotation torque between the pinion shaft and differential case in a differential state.
In this differential gear, when the rotation resistance is the same for both the left and right wheels, the pinion gear, side gears, pinion shaft, and pressure rings all rotate integrally with the differential case and the left and right wheels rotate at the same speed. However, when the rotation resistance is different for the left and right wheels, the pinion revolves while engaging with the side gear using part of the torque that acts on the differential case, and the differential gear switches to either a differential state in which the rotation speed of the wheel on the side of less resistance is faster than the rotation speed of the wheel of more resistance, or a limited differential state in which the relative rotation between the side gears and the differential case is limited and part of the rotation torque that works on the differential case is distributed to the wheels on the side with greatest resistance.
More specifically, when the rotation resistance of the left and right wheels is different, the operating force that acts in a direction to increase the clearance between the pressure rings in accordance with the relative torque between the pinion shaft and the differential case, acts on the pressure rings. However, when the operating force of this operating means is smaller than the urging force brought about by the urging means that attempts to reduce this clearance, the clearance does not change and an ordinary differential state is obtained. However, when it is greater than the urging force, the clearance between the pressure rings increases accordingly, the clutch plates in the clutch means are frictionally abutted, and the differential between the left and right axles is limited.
Now, the urging means for this differential gear exerts a force on the pressure rings in a direction that narrows the clearance without going through the clutch means. Even when a strong urging force is set for this urging means, the clutch plates of the clutch means will not abut directly, and the timing of the switching from the differential to limited differential state depends only on the operating power provided by the operating means and the urging force provided by the urging means. Therefore, by setting the urging force of the urging means at an appropriate level, an accurate differential state can be obtained for low speed turning and an accurate limited differential state can be obtained for high speed, heavy load turning.
The differential gear according to a second embodiment of the invention comprises: a differential case that is rotated by a driving force from the engine around the left and right axles; pressure rings accommodated in said differential case so as to be able to move freely in the direction of the axles but unable to rotate relatively; a pinion shaft provided at right angles to the axles with an end thereof being held between said pressure ring and the face, opposing to the pressure ring, of the differential case facing to the pressure ring; at least one pair of pinion gears provided rotatably on said pinion shaft; a pair of side gears, each being placed at the end of said left and right axles so as to be unable to rotate relatively, that sandwich the pinion gears and engage with the pinion gears on the opposite sides thereof; clutch means arranged on the opposite sides to the opposing faces of said pressure ring and that can limit the differential movement between the left and right axles by limiting the relative rotation of the differential case and side gears; urging means that urges the pressure rings in such directions as to decrease the clearance between the pressure ring and the face opposing to the pressure ring of the differential case, without use of said clutch means; and operation means that operates the pressure rings against the urging force exerted by the urging means in such directions as to increase the clearance between the pressure ring and the face opposing to the pressure ring of the differential case in accordance with an increase in the relative rotation torque between the pinion shaft and differential case in a differential state.
In this differential gear, basically, when the rotation resistance of the left and right wheels is the same, the pinion gear, side gears, pinion shaft, and pressure rings rotate integrally with the differential case and the left and right wheels rotate at the same speed. However, when the rotation resistance of the wheels is different, the pinion revolves while it engages with the side gears using part of the torque that acts on the differential case. There is then a switch to either a differential state in which the rotation speed of the wheels with least rotation resistance is greater than the rotation speed of the wheels with greatest rotation resistance or a limited slip differential state in which the relative rotation of the side gears and differential case is limited using the clutch means and part of the torque that acts on the differential case is distributed to the wheels with greatest rotation resistance.
More specifically, if the rotation resistance of the left and right wheels is different, when the operating force of the operation means, which acts to increase the clearance between the pressure ring and the face opposing to the pressure ring of the differential case according to the relative torque between the pinion shaft and differential case, is smaller than the urging force of the urging means, which acts to reduce this clearance, the clearance will not change and a normal differential state will be obtained. When the operating force is greater than the activation force, the clearance between the pressure ring and the ring opposing face will increase accordingly, the clutch plates in the clutch means will be frictionally abutted, and the differential between the left and right wheels will be limited.
In the urging means in this differential gear, the pressure ring is urged in such a manner that the clearance is reduced without using the clutch means. Even when a high urging force is set for the urging means, the clutch plates in the clutch means will not be frictionally abutted, and the timing of the switching from the differential to limited differential state depends only on the operation force in the operation means and the urging force in the urging means. Therefore, by setting the urging force to an appropriate level, an accurate differential state can be obtained when turning at low speeds and an accurate limited differential state can be obtained when turning at high speeds and under heavy loads.
Furthermore, only one clutch means need be installed in this differential gear. This improves the ease of assembling the differential gear and enables the incorporation of a clutch means in a side where space is available. This means that a small differential gear can be built while the size of clutch plate can be maximized to increase the friction abutting force. Therefore, this type of differential gear can be used in engines where there are major restrictions on the form and size of the differential case and in automobiles with transverse engines. When only one clutch means is installed, the relative rotation between one side gear and the differential case is directly limited by the clutch means and the relative rotation between the other side gear and the differential case is limited by the clutch means via the pinion and the first side gear.
The urging means may be provided in the vicinity of the ends of the pinion shafts within the pressure rings. The urging means can be disposed in any position but the operating force that acts to increase the clearance normally acts on the pressure rings from the ends of each pinion shaft. Therefore, by placing the urging means near the ends of the pinion shafts, the operating force can act in a balanced way with the urging force. This is preferable since it improves the ease with which pressure rings can slide.
Alternatively, the urging means may be provided between the ends of the pinion shafts adjoining the pinion shafts within the pressure rings. The ends of the pinion shafts are held between the pressure rings or between the pressure ring and the ring opposing face and therefore, if the urging means is located further outside in the radial direction than the end of the pinion shaft, the size of the pressure rings and differential case will be increased by that amount. Therefore, it is preferred that the urging means be provided between the adjoining ends of the pinion shaft within the pressure rings in terms of reducing the sizes of the pressure rings and differential cases. This configuration also allows the ends of the pinion shafts to extend inside the differential case and increases the area of the contact between the pinion shaft and the pressure rings. This effectively prevents friction between and damage of these two members caused by the relative torque acting between the pinion shaft and the pressure rings and improves the durability of the differential gear.